In U.S. Pat. No. 3,099,672, a method is described for reacting halogen- or alkoxysilanes with sodium hydride at a temperature of 175.degree. to 350.degree. C. By employing this method, which is applicable only for silanes and not for organopolysiloxanes, the halogen or alkoxy group is replaced by a hydrogen atom. The yields obtained are variable.
In accordance with U.S. Pat. No. 3,099,672, the sodium hydride is used as such, as a suspension in mineral oil or other high-boiling, inert hydrocarbons or in the form of a coating on sodium chloride particles. The need to use relatively high temperatures makes the method more expensive and frequently leads to unwanted by-products.
In U.S. Pat. No. 3,535,092, a method is described for the reaction of compounds, which have Si-halogen groups, with sodium hydride. This reaction proceeds already at room temperature or at temperatures raised only moderately. This is accomplished by the use of special solvents, such as hexaalkyl phosphoramides, octaalkyl pyrophosphoramides and tetraalkylurea. The alkyl groups in the solvent contain 1 to 4 carbon atoms. It may, moreover, be inferred from U.S. Pat. No. 3,535,092 that these solvents are said to have a catalytic effect. The method is applicable for halogensilanes as well as for halogensiloxanes. Alkoxysilanes do not react under the same conditions. Since the solvents have high boiling points, the products of the method, which contain SiH groups, are distilled from the reaction mixture. Consequently, the applicability of this method is restricted to those SiH-containing silanes or siloxanes, which can be distilled off at a technically justifiable expenditure and without thermal decomposition. Removal of the aforementioned solvents by washing them out is not readily possible. On separating the solvent with water, the excess sodium hydride forms sodium hydroxide, which leads in the aqueous medium to the formation of SiOSi bonds, hydrogen being split off. However, since the solvents named in this patent are physiologically objectionable, a complete separation of the Si-containing products of the method from the solvents is a necessary prerequisite for most applications.
Instead of sodium hydride, it has been suggested to use complex hydrides, such as sodium aluminum hydride, to exchange Si-halogen groups for SiH groups. For example, a method is described in the German Auslegeschrift No. 1,085,875 for preparing polysiloxane hydrides of the general formula ##STR1## wherein n is a whole number, especially one form 1 to 5, and R represents hydrogen, alkyl or aryl. Pursuant to this method, polysiloxanes of the general formula ##STR2## in which X represents chlorine, alkyl or aryl and n has the above meaning, are reacted with lithium aluminum hydride. In accordance with the examples of the German Auslegeschrift No. 1,085,875, 1,7-dichlorooctamethyltetrasiloxane is converted with lithium aluminum hydride in tetrahydrofuran in 74% yield into 1,7-dihydrooctamethyltetrasiloxane. The conversion of 1-chloropentamethyldisiloxane into 1-hydrogenpentamethyldisiloxane with lithium aluminum hydride is said to proceed in a 57% yield.
As is evident from the "Polish Journal of Chemistry" 53 (1979) 1383 to 1386, a reproduction of the method of the German Auslegeschrift No. 1,085,875 has shown that SiOSi bonds are split in competition with the exchange of Si-halogen groups, so that silanes, such as the gaseous and easily ignitable dimethylsilane, are formed to a considerable extent as by-product. This is in agreement with the information in "Chemie und Technologie der Silicone" (Chemistry and Technology of the Silicones) by W. Noll, Verlag Chemie, 1968, page 206. It is stated there that siloxane bonds are converted into alumosiloxane bonds by the action of lithium aluminum hydride in ether on linear polydimethylsiloxanes, silanes being split off. This splitting reaction is practically quantitative with silicate esters. The splitting of SiOSi bonds by the action of lithium aluminum hydride may also be inferred from the Zeitschrift fur Naturforschung, 10 b, (1955) 423 to 424.
A similar method, in which trisodium aluminum hexahydride is used as complex aluminum hydride, is described in the German Auslegeschrift No. 1,568,255. The splitting of SiOSi bonds is a competing reaction, even when this complex hydride is used.